INVESTIGATION OF FAULTS IN THE WESTERN ALASKA RANGE

Investigation of fault offsets in the western Alaska Range is resulting in predicted as well as un-anticipated results. At last year’s GSA Cordilleran meeting Hults and Wilson (2009) hypothesized that there is a major fault offsetting the Talkeetna Mountains from similar rocks found to the west along the western Alaska Range. The Jurassic phase of the Alaska-Aleutian Range batholith, cropping out in the Talkeetna Mountains and west of Cook Inlet, appears offset up to 200 km along an unrecognized fault. The Talkeetna Arc Jurassic plutons, associated volcanic and sedimentary rocks, and older basement rocks appear to be offset along the Castle Mt. Fault and along the new proposed segment running through the Chakachamna Lake Region. Alaska State Division of Geological and Geophysical survey geologists’ recognition of fault near the Capps Glacier and sediments deposited in an associated Eocene basin support this hypothesis (Gillis and others, 2009). Field examination of an area along the Neacola River reveals fault gouge west of, and on strike with, the Capps Glacier fault, supporting the hypothesis of a through-going fault. The fault gouge lies along the Telaquana fault proposed by Haeussler and Saltus (2005) based on aeromagnetic patterns. However, detrital zircon age populations found in Late Jurassic to Early Cretaceous sedimentary rocks north and south of the postulated fault contain similar population distributions and a significant Lower Jurassic population suggesting a continuous basin sourced from primarily the Jurassic phase of the Talkeetna Arc. These detrital zircon data do not support major offset along the proposed fault. Thus far, the evidence suggests faulting through the Chakachamna Lake area with limited offset - tens of kilometers, not 200 kilometers.